This invention relates to an optical isolator device having an optical axis for permitting an incident light beam in a direction along the optical axis to pass therethrough and for preventing another incident light beam in the opposite direction from passing therethrough.
As is well known in the art, an optical isolator device is used in, for example, a laser diode (LD) module which comprises a semiconductor laser diode and an optical fiber. The optical isolator device is disposed between the semiconductor laser diode and the optical fiber. An optical axis of the optical isolator device is coincident with an optical axis of the semiconductor laser diode and an optical axis of the optical fiber without offset. The semiconductor laser diode emits a light beam which is incident as an incident light beam onto an incident end of the optical isolator device. The optical isolator device permits the incident light beam to pass therethrough. Accordingly, the incident light beam emits from an outgoing end of the optical isolator device as an outgoing light beam which is, in turn, incident onto an incident end of the optical fiber. A part of the outgoing light beam is reflected on the incident end of the optical fiber as a reflected light beam. The remainder of the outgoing light beam is transmitted through the optical fiber as a transmitting light beam. A ratio of intensity of the transmitting light beam to intensity of the outgoing light beam is called a coupling coefficient. The reflected light beam is returned as a return light beam to the outgoing end of the optical isolator device in the opposite direction of the outgoing light beam. The optical isolator device prevents the return light beam from passing therethrough in the opposite direction, Therefore, the reflected light beam is cut off by the optical isolator device to inhibit the semiconductor laser diode from receiving the reflected light beam. As a result, it is possible to make the semiconductor laser diode operate stably without appearance of noise.
A conventional optical isolator device comprises a magnet having a rectangular channel which has an inner surface and a first central axis. An optical isolator element is a body with a rectangular section and has an outer surface and a second central axis as an optical axis. The rectangular optical isolator element is fitted and fixed in the rectangular channel by adhesive filled up in a gap around the optical isolator element and between the outer surface of the optical isolator element and the inner surface of the rectangular channel in a condition that the first central axis and the second central axis are parallel without offset.
From the reason in production of the magnet, the rectangular channel is formed round at the four corners, for example, with about 0.1 mm radius of curvature. On the other hand, the rectangular optical isolator element can be formed with a clean and sharp corners by use of a dicing saw. Therefore, the rectangular channel is formed slightly larger, for example, by 0.1 mm than the optical isolator element. As a result, the thickness of the adhesive filled up in the gap is disadvantageously large from the view point of the adhering intensity of the adhesive. Therefore, when the adhesive in the gap is exposed in an environment of a high temperature and/or a high humidity and is deformed thereby, the optical isolator device is defective in that the second central axis inclines against the first central axis by the deformation of the adhesive. The inclination causes a deterioration of a coupling coefficient between the optical isolator device and an optical fiber in an LD module.